Injections are made through bone tissue for diagnostic and for therapeutic purposes, and in particular for obtaining local anesthesia of the teeth on which a dentist is about to operate.
In dental anesthesia, the conventional solution consists in using a conventional syringe to inject a predetermined quantity of liquid anesthetic into the patient's gum at a certain distance from the toot to be treated (block, conduction, and para-apical anesthesia). As it diffuses from the point of injection, the anesthetic anesthetizes the surrounding region which includes the tooth to be treated. This solution suffers from the following drawbacks: the injection is performed blind; there is a period of latency between the injection and the tooth to be treated becoming anesthetized; more anesthetic is used than would be necessary for anesthetizing the tooth only; the patient is left with a disagreeable sensation in the mouth which lasts for a long time after the dentist has finished working on the tooth; etc. . .
Proposals have already been made to remedy these drawbacks by using an intramedullary anesthetic which is injected into the porous (ethmoid) bone of the upper or lower jaw in the immediate vicinity of the tooth to be treated. However, in this case it is necessary to pass through the hard shell or osseous cortex of the bone before injecting the anesthetic. French published patent specification No. 2 457 105 describes a syringe suitable for performing this operation. The syringe described in that French patent rotates the needle to enable it to pass through the cortex, with the injection per se then being performed along the hollow needle by drawing a moving trigger towards a fixed rest, with the moving trigger operating a piston which displaces a carpule type cartridge or store of anesthetic. The needle and the carpule are rotated either by a mechanical drive of the type available next to a dentists' chair and generally used for driving a dental drill, or else by means of a small motor disposed inside the syringe and connected to a suitable supply of electricity.
The method and the apparatus described in the above-mentioned patent specification have considerably improved dental surgery in that, relative to conventional anesthesia, they provide: greater freedom in selecting the site at which to perform the injection; greater latitude in the angle of approach of the needle; and a relatively small quantity of anesthetic is actually injected into the porous bone.
However, this syringe suffers from drawbacks. In particular, there is a danger of the channel along the hollow needle becoming obstructed, for example by a mixture of bone dust and blood as it passes through the cortex. This may happen if the speed of rotation is too low, or if the force with which the needle is pressed against the bone is too high. When the channel along the needle has become obstructed, the pressure exerted on the anesthetic must be increased. This has two unfortunate consequences: the extra effort required of the practitioner causes the needle to tremble and thus spoils the accuracy with which the needle is located and also spoils the evenness of the rate at which anesthetic is injected. In particular, the sudden rush of liquid under pressure into the porous bone which occurs immediately after the obstruction has been ejected is particularly painful for the patient.
Preferred embodiments of the present invention mitigate these drawbacks by automating operation of the instrument in such a manner as to ensure that the anesthetic (or more generally any liquid to be injected into a bone) is injected evenly. Preferably, the electrical power supply to the instrument is completely independent from the dentists' chair so as to avoid the need for any connecting wire which could hinder the manipulations performed by the dentist.
Further, and more generally, all prior known syringes have had to imitate the cylindrical shape of a conventional syringe. However, this shape is poorly adapted to the work of injecting through bone since the syringe should be capable of being firmly gripped in the hand in order to ensure that positional accuracy is maintained throughout the injection operation. Preferred embodiments of the present invention also improve the accuracy with which the instrument can be manipulated by providing an apparatus having a good hand hold.